Magnetron power output control



1961 o. A. WILBUR 2,995,715

MAGNETRON POWER OUTPUT CONTROL Filed June 5, 1958 /n venfor Donald A W/Jbur His Af/omey.

United States Patent 2,995,715 MAGNETRON POWER OUTPUT CONTROL Donald A.Wilbur, Scoti-a, N.Y., assignor to General Electric Company, acorporation of New York Filed June 3, 1958, Ser. No. 742,460 8 Claims.(Cl. 332-) This invention relates in general to magnetron modulation andhas particular reference to means for controlling the power output ofmagnetron oscillators. This applL cation is a continuation-in-part of myapplication Serial Number 298,227, filed July 11, 1952, entitledMagnetron Power Output Control" and assigned to the assignee of thisapplication.

Discharge devices of the magnetron type, particularly of the travelingwave type, have found extensive use as oscillators because of theirability to produce large amounts of high frequency power at relativelyhigh efiiciencies. For many applications it is desired to control oramplitude modulate the power output easily and conveniently withoutchanging the magnetic field or the voltage of the direct current powersupply. It is furthermore desirable to utilize a control voltagerequiring a minimum amount of power from the control voltage source.

It is accordingly a primary object of my invention to control the poweroutput of a magnetron oscillator by an applied control voltage.

It is a further object of my invention to provide a magnetron modulationcircuit requiring but a small amount of control power.

It is a further object of my invention to provide a simplifiedmodulation circuit for a magnetron oscillator which does not requirevariation of either the magnetic or main electric field.

In accordance with my invention, an anode control section comprising oneor more segments or vanes of a multi-segment magnetron anode structureis provided with a separate direct current connection and aunidirectional bias or control voltage is connected in series betweenthe control section and the anode voltage source to control the poweroutput in the high frequency resonant circuit. The control section maybe a part of the resonant circuit or it may be insulated from theresonant circuit. To the extent the total voltage, i.e., the anodevoltage plus the bias voltage, is increased between the control sectionof the anode and the cathode, the power output of the magnetron isincreased. Only a fraction of the energy utilized to accelerate thespace charge and thus increase the power output is provided by thecontrol voltage source, however, since the control voltage representsonly a fraction of the total voltage impressed on the control section.Loading of the control voltage source is further minimized since verylittle of the discharge current is collected by the control anodesegment or segments despite their higher voltage. Instead, the rotatingspace charge, which tends to follow equipotential electric field lines,first contacts and is collected by the lower potential segmentsconnected to the power output circuit.

The operation just described in which the space charge becomes bunchedand makes several excursions past the anode gaps in synchronism with thehigh frequency wave on the anode structure is characteristic ofmagnetron devices and systems which have become known as traveling wavetype magnetrons. In these systems energy is transferred from the spacecharge to the anode structure by the motion of the bunches past theanode gaps. In magnetron oscillators of the negative resistance type theoperation is essentially non-synchronous and the energy transfer isaccomplished by the collection of electrons on the anode surface whichcomplete the excursion from the cathode to the anode in a fraction of acycle at the frequency of operation.

The features of the invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its 0rganization and method of operation together withfurther objects and advantages thereof may best be understood by thefollowing description taken in connection with the accompanying drawingin which- FIG. 1 represents magnetron apparatus embodying my invention;

FIG. 2 is a set of power output characteristic curves;

FIG. 3 is a semi-schematic view of magnetron apparatus representing amodification of my invention;

FIG. 4 is an elevational view in section of an interdigital magnetronsystem embodying my invention; and

FIG. 5 is a sectional view taken along the line 55 of FIG. 4.

Referring now to FIG. 1 of the drawing an electron discharge device ofthe magnetron type is shown having an evacuated envelope 1 which issuitably made of glass and within which are mounted a cathode 2 andanode segments 3 and 4. These segments define between them a cylindricalspace charge chamber coaxial with the cathode and are representative ofthe usual anode segment array of a conventional magnetron of thesplit-anode or multi-vane type. The cathode 2, which may suitably takethe form of a helix of tungsten wire having a thermionic cmissivecoating thereon, such as barium oxide, is supported at its ends byconductors 5 sealed through the base of the envelope 1 to provideexternal terminals. The anode segments 3 and 4 are respectivelysupported by parallel conductors 6 and 7 which are also sealed throughthe envelope base.

The parallel conductors 6 and 7 are capacitively terminated at theirexternal ends to define a resonant output circuit. Its frequency isgoverned by the spacing and length of the conductors 6 and 7, thecapacitance of the terminating capacitor 8 connected across their endsbeing sufliciently large to present substantially zero impedance at theresonant frequency. The anode segments 3 and 4 are thus isolated forapplication of different direct current potentials.

In operation a magnetic field is provided in the space charge chamber incoaxial relationship therewith, the means for producing the field beingschematically indicated as a solenoid 9 although, of course, magnetmeans may be substituted. A suitable source of heater voltageconventionally represented as a battery 10 is connected between thecathode terminals 5 to provide the proper cathode operating temperaturefor emission of the electrons constituting the magnetron space charge.

Unlike conventional magnetrons, however, and in accordance with myinvention, all of the anode segments of the anode arrangement, in thiscase the two segments 3 and 4, are not operated at the sameunidirectional voltage with respect to the cathode. While both segmentsare connected in circuit to the positive terminal of the anode voltagesupply 11, conventionally represented as a battery having its negativeterminal connected to the cathode, an additional voltage is connected incircuit with segment 3. This added signal is supplied from an additionalunidirectional control voltage source 12 connected in series with source11 with its positive terminal connected to conductor 6 of the outputcircuit and its negative terminal connected to the positive terminal ofvoltage source 11. Segment 3 accordingly serves as the control electrodeof the anode array.

The direct current isolation of the anode segments 3 and 4 must bemaintained for operation of the magnetron according to my invention andaccordingly the output load, schematically represented as a resistor 13,is connected across the resonant circuit conductors 6 and 7 through thedirect current blocking or isolating capacitor 14.

While the desired modulation of the control voltage may actually beprovided by a source 12 of alternating signal voltage superimposed on apositive bias voltage, it is important, for proper operation, that theinstantaneous applied control section total voltage should at all timesbe at least as high as the anode voltage from the source 11. Thesignificance of this requirement for maintaining the controlcharacteristics generally desired is analogous to the desirability ofpreventing the grid current in a conventional triode. In the triode thecontrol electrode should be negative with respect to the cathode; in thepresent invention the control section should be positive with respect tothe anode. Accordingly, it is significant that the additional voltagesupplied from the control voltage source 12 is not connected between thecathode and all of the anode segments coupled to the output circuit.

As is well known in magnetron operation, the electronic space charge isacted upon by the radial electric field and the axial magnetic field inthe space charge member so that it assumes an average angular velocityabout the cathode in a given direction. Assuming the usual 1r modeexcitation of a traveling wave magnetron oscillator, the electronstraversing a gap between adjacent segments at an interval when they arein phase with a fringing alternating electric field between the segmentsestablished by the initial excitation of the tuned circuits reinforcethe oscillations by giving up part of their kinetic energy to thealternating electric field. Upon losing some of their energy theelectrons move toward the anode to regain energy and thus eventually arecollected on the anode after an excusion past a number of gaps andusually after a substantial number of revolutions in the interactionspace. In this way the space charge envelope is distorted to assume aspoke-shaped form, each spoke corresponding to a region of in-phaseelectrons having an average angular velocity synchronous with the highfrequency electric field of the anode assembly. Since this averageangular velocity is dependent upon the ratio of the radial electricfield to the axial magnetic field within the space charge chamber anincrease in the radial electric field tends to increase the averagevelocity above synchronism, thus cause a greater interchange of energyfrom the electrons to the high frequency fields.

Accordingly, upon the addition of the control voltage to the anodevoltage on the control segment 3, the space charge spoke passing underthe control anode segment 3 is accelerated and tends to exceed thesynchronous frequency. As a result, more energy is transferred to thehigh frequency alternating fields and the power delivered to the load 13and 14 increases. Moreover, since the rotating space charge tends tofollow equipotential lines most of the electrons of the space chargespokes which have given up their energy or a portion of their energy tothe fringing fields between the anode segments are ultimately collectedon the segment having the lower potential. Accordingly, most of thecollected electron current flows from the cathode through the outputsegment 4 and returns to the anode voltage source 11 without loading thesource 12 of control voltage. As the control voltage is increased thedistortion of the static electric field equipotential lines isincreased, and hence the loading of the control voltage by collectioncurrent increases very little with power output.

As may be noted from FIG. 2, the power output, which in this case is thepower dissipated in the resistor 13, increases linearly with theincrease of the positive control voltage Ec superimposed on the anodesupply. Curve A, which shows Zero power output when the control voltagefrom source 12 is zero, is obtained when the voltage of the anode supply11 is just sufliciently high to initiate oscillation. The voltage of theanode supply 11 is adjusted as desired to determine the control voltageat which zero power is obtained, the magnitude of the superimposedcontrol voltage determining the maximum power level. Linear curves B andC of FIG. 2 illustrate this effect.

Since the control voltage from sources 12 and 12 is also normally but asmall fraction of the anode voltage from source 11, the increased energyimparted to the space charge spokes passing under the control electrodesis derived mainly from the anode voltage source 11.

Referring now to FIG. 3, a power amplifier utilizing a neutral anodesegment as the control electrode is illustrated. The magnetron comprisesan elongated cathode 15 surrounded by an anode assembly 16 which definesa cylindrical space charge chamber coaxial with the cathode. Only threeanode segments are employed in the embodiment illustrated, two of thesesegments 17 and 18 being the output segments to which a tuned outputcircuit 19 is connected. These output segments each subtend an angle ofapproximately to the space charge chamber axis, thus corresponding inwidth with the vanes of a conventional four-vane magnetron. The thirdsegment 20 subtends an angle of almost and is normally employed as aneutral element in a magnetron oscillator, being coupled only to aneutral point, so far as high frequencies are concerned, of the resonantoutput circuit 19. This anode segment, called a neutrode, maintains theradial electric field in the space charge chamber, and is profitablyemployed to replace a pluralityin this case twoof the anode outputsegments. Since the positioning of a full array of anode segments isoften difiicult, particularly where a large number of segments aredesired for higher frequency operation, a neutrode may replace all butone or a few pairs of high frequency coupled segments. A furtherdiscussion of the advantages of the magnetron neutrode construction andan explanation of its operation is presented in my Patent No. 2,462,698,issued February 22, 1949, and assigned to the assignee of the presentinvention.

As further shown in FIG. 3, the tuned output circuit 19, to which theoutput anode segments 17 and 18 are connected, is a resonant loopcomprising a section of parallel conductor transmission linesshort-circuited at one end. An output load 21, schematically representedas a resistor, is suitably connected across the parallel conductors at apoint where a standing wave voltage exists. The midpoint of the loop,which is its high frequency neutral point, is connected to the positiveterminal of the anode voltage supply 23 whose negative terminal isconnected to the cathode. The neutrode 20, since it is designed to beneutral so far as the output frequencies are concerned is not coupled toa tuned circuit but is connected directly to a positive tap on a controlvoltage source 23 whose negative terminal is connected to the positiveterminal of the anode voltage supply 22. The control voltage sourcecorresponds to the signal source 12 of FIG. 1. Modulation of the poweroutput may be accomplished by mechanical movement of the tap orpreferably by a source of alternating signal voltage 23' having anamplitude less than the magnitude of the direct current voltage 23.

Thus it may be seen that, as in the case of FIG. 1, a radial electricfield is impressed between the cathode 15 and the anode assembly 16 withthe field intensity under the control neutrode 20 being capable of beingincreased above that supplied by the anode voltage source 22 due to thesuper-position of a control voltage. As previously discussed, when asuitable axial magnetic field is supplied in the space charge chamber,the addition of the control voltage to the anode supply voltage providesadditional velocity to the rotating space charge to produce a greaterpower output. Since the radial electric field is distorted from symmetrydue to the unbalance caused by the control voltage, the electroniccurrent from the cathode accompanying the increased power output iscollected by the output anode segments 17 and 18 which are at a lowerD.-C. potential and loading of the control voltage source is avoided.

It should be understood, of course, that the specific discharge deviceillustrated in FIG. 1 is by way of example only, as other traveling wavemagnetrons may be suitably operated in apparatus embodying my inventionso long as the control and power output portions of the anode system areisolated for direct current or low frequency voltages. Greater numbersof vanes or segments may be employed, and for the higher frequencyranges the resonant output circuit may be entirely contained within thedevice envelope.

In FIGS. 4 and 5 I have shown my invention embodied in an interdigitaltype magnetron which may be manu factured readily with a substantialnumber of vanes and is, accordingly, suited to higher frequencyoperation. As illustrated in FIGS. 4 and 5 the magnetron device includesa generally cylindrical envelope made up of hollow cylindricalinsulators 24, 25 and 26 preferably of ceramic material, a pair ofannular anode terminals 27 and 28 and a pair of disk-like end caps 29and 30. The insulator 25 mutually spaces the anode terminals 27 and 28while the insulator 24 separates end terminal 29 from anode terminal 27and insulator 26 spaces end terminal 30 from the anode terminal 28. Aswill be readily appreciated the metal terminals and insulating spacersare suitably bonded together during fabrication of the device to form avacuum tight envelope. The high frequency or anode structure is providedby two sets of anode sections 31 and 32 supported in a cylindrical arraywith the sections of each set alternately arranged and with the sectionsof one set supported from anode terminal 27 and the sections of theother set supported from the anode terminal 28.

A generally cylindrical emitting cathode sleeve 33 is supportedcentrally within the array of anode sections from the end caps 29 and 30by means of conducting posts 34 and 35. As will be readily appreciatedthe cathode is preferably coated with a suitable oxide emissionenhancing coating and provides a source of electrons within theinteraction space defined between the array of anode sections 31, 32 andthe cathode 33. The supporting post 35 is made hollow to provide for thepassage of a heater supply conductor 36 which is connected to one end ofa spiral heater 37 supported within the cathode cylinder, the otherterminal of the heater being connected with the cathode support 34.Conductor 36 is sealed through the end cap 30 by an insulator 38 andconnected with one terminal of a suitable heater supply voltage source36, the other terminal of which is connected to the cathode terminal 30.A generally axial magnetic field in the interaction space is produced bysuitable means which has been illustrated schematically by the polepieces 39.

In accordance with an important aspect of the present invention and inaccordance with principles already discussed in this application, oneset of anode segments 31 is maintained at a positive voltage withrespect to the anode segments 32. As illustrated in the drawing, theanode terminal 28 with which the anode segments 32 are connected ismaintained at a positive direct current voltage by means of a battery 40connected between the cathode terminal 30 and the anode terminal 28. Theanode segments 31 are maintained at a positive voltage with respect tothe anode sections 32 by a biasing battery 41 connected between anodeterminal 28 and the anode terminal 27 through a circuit includingconductor 42 and also an alternating modulating voltage 43. Aspreviously discussed, this voltage is of an amplitude smaller than themagnitude of the direct current voltage 41 so that the anode or controlanode segments 31 are always positive with respect to the remaininganode sections 32.

The output circuit for the magnetron device may be in the form of agenerally rectangular wave guide suitably apertured to receive the anodeterminals 27 and 28. In order to maintain the direct current isolationrequired for biasing of the anode segments 31 with respect to the anodesegments 32 the high frequency connection between the anode terminal 27and the upper wall of the wave guide is completed through a dielectricwasher 44 to provide a bypass capacitor.

The operation and advantages of my invention as applied to theembodiment of FIGS. 4 and 5 are believed to be understood from theearlier discussion in the application. It is to be noted that thecontrol of modulating voltage is only a small portion of the totalvoltage of the control anode segments 3-1 and, accordingly, only a smallamount of control power is required. Also, due to the movement of thespace charge outward toward the structure as the energy is absorbed fromthe electrons the majority of the electrons are collected, not on thehigher voltage anode segments but on the intermediate lower voltageanode segments 32. Thus, dissipation of power in the control circuit iskept at a minimum and effective control or modulation of the outputpower is readily accomplished.

While I have shown particular embodiments of my invention, it will, ofcourse, be understood that numerous modifiactions may be made by thoseskilled in the art without actually departing from the invention. I,therefore, aim in the appended claims to cover all such equivalentvariations as come within the true spirit and scope of the foregoingdisclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A magnetron system including a magnetron of the traveling wave typehaving a plurality of anode segments surrounding a centrally disposedcathode, means for applying a unidirectional voltage between saidcathode and one of said plurality of anode segments to maintain saidsegment at a fixed positive potential with respect to said cathode andmeans for impressing an adjustable voltage to said one of said anodesegments positive with respect to another of said anode segments tocontrol the power output of said magnetron system.

2. A magnetron system including a magnetron of the traveling wave typehaving a plurality of anode segments surrounding the centrally disposedcathode, means for applying a unidirectional voltage between saidcathode and one of said plurality of anode segments to maintain saidsegment at a fixed positive potential with respect to said cathode andmeans for impressing a variable magnitude modulating voltage to said oneof said anode segments positive with respect to another of said anodesegments to control the power output of said magnetron system.

3. A magnetron system including a magnetron of the traveling wave typehaving a plurality of anode segments surrounding the centrally disposedcathode, and means for impressing a direct current voltage component offixed magnitude and an alternating current modulating voltage having amaximum amplitude less than the magnitude of said direct current voltagecomponent between said one of said anode segments and another of saidsegments to control the power output of said magnetron system and meansfor impressing a direct potential of magnitude less than the minimumvalue of the sum of said voltages to said other of said anode segments.

4. A magnetron system including a magnetron device of the traveling wavetype including a cathode, a pair of mutually insulated anode terminals,an array of anode segments surrounding a centrally disposed cathode withalternate segments connected with a different one of said pair ofmutually insulated anode terminals, means for applying a unidirectionalvoltage of fixed magnitude between said cathode and one of said anodeterminals, means for impressing a unidirectional voltage of adjustablemagnitude between said one of said anode terminals and said cathode tocontrol the power output of said magnetron system and means forimpressing a direct potential to the other anode terminal of lessermagnitude than the minimum sum of said voltages.

5. A magnetron system including a magnetron device of the traveling wavetype including a pair of mutually insulated anode terminals, a cathode,a generally cylindrical array of anode segments surrounding said cathodewith alternate segments connected respectively with a different one ofsaid anode terminals, means for impressing a unidirectional voltage offixed magnitude between said cathode and the anode segments connectedwith one of said anode terminals and means impressing a unidirectionalbut variable amplitude component of modulating voltage having a minimumvalue of magnitude greater than said voltage of fixed magnitude betweenthe anode segments connected with one of said anode terminals and theanode segments connected with the other of said anode terminals tocontrol the power output of said magnetron system.

6. A magnetron discharge device of a type having a plurality of anodesegments surrounding a centrally disposed cathode in a cylindrical arraycoaxial therewith, said anode segments cooperating with said cathode todefine a substantially annular space charge region, a resonant outputcircuit coupled to selected segments of said plurality of segments,means for maintaining said selected segments at a positive operatingpotential with respect to said cathode, and means for controlling thepower in said output circuit comprising means for impressing a positivevoltage larger than said operating potential between said cathode andone of said anode segments other than said selected segments.

7. An oscillator for producing high frequency energy comprising amagnetron discharge device of the type hav ing an anode including aplurality of output segments and a control segment surrounding acentrally disposed cathode in a cylindrical array coaxial therewith,said anode segments cooperating with said cathode to provide asubstantially annular space charge region, an output circuit coupled tosaid output segments, means for connecting a source of direct currentvoltage between said output segments and said cathode to maintain saidoutput group of anode segments at a positive operating potential withrespect to said cathode, and means for connecting a source ofunidirectional control voltage in cumulative series between said controlsegment and said direct current voltage source to maintain said controlsegment at a potential greater than said operating potential segment inorder to control the magnetron power output.

8. A high frequency magnetron oscillator comprising a magnetrondischarge device of the type having a plurality of anode segmentssurrounding a centrally disposed elongated cathode to define acylindrical space charge chamber coaxial therewith, one of said anodesegments subtending a substantially greater angle with respect to saidspace charge chamber axis than the other segments of said plurality ofsegments, an output circuit connected to said other segments tuned tothe desired output frequency, means for supplying a positive potentialwith respect to said cathode to said other segments through a highfrequency neutral point of said output circuit and to said one segment,and means for increasing the potential applied to said one segment abovethe potential applied to said other segments for increasing the power insaid output circuit.

References Cited in the file of this patent UNITED STATES PATENTS2,220,968 Link Nov. 12, 1940 2,292,073 Hansell Aug. 4, 1942 2,810,095Peters Oct. 15, 1957 FOREIGN PATENTS 736,507 Germany May 13, 1943851,554 France Jan. 11, 1940 OTHER REFERENCES Kilgore: MagnetronOscillators," August 1936, IRE Proceedings, pp. 1140-1147 relied on.

